Circuit designers often require transistors with different characteristics e.g. high performance, low leakage and/or multiple resistances to high voltage. These different characteristics can be achieved with different gate dielectric film properties and/or thicknesses.
The most common method of forming several thicknesses of gate oxide on the same chip is resist on oxide. However, this method becomes inappropriate when applied to form more than the standard "dual gate" devices.
For example, if a designer wants three gate oxide thicknesses, a standard resist on oxide approach would require 2 intermediate HF etches, 3 oxidations and 2 block level masks. This approach is not only expensive but also forms edge "divots" and planar recess in shallow trench isolation (STI) areas as a consequence of additional oxide etching.
Another method of forming several thicknesses of gate oxide on the same chip is by using nitrogen implantation. The disadvantage of this approach is that the gate thickness differences are dependent on nitrogen implant dose. It is known that too high nitrogen implant dosages degrade gate oxide reliability. Therefore, there is a limitation on gate oxide thickness options for various combinations of devices.